In recent years, hybrid electric vehicles (HEVs), electric vehicles (EV), and fuel cell vehicles are under development, some of which are already manufactured and sold. These vehicles are also called electrically-powered vehicles and are attracting attention because of growing interest in the environmental friendliness and the high fuel economy thereof. The electrically-powered vehicles require power supply devices capable of being charged and discharged. The power supply devices are electric devices including secondary batteries such as lithium-ion and nickel-hydrogen secondary batteries and electric double-layer capacitors. The lithium-ion secondary batteries, in particular, are preferably used in electrically-powered vehicles because of the high energy density thereof and the high durability against repeated charge and discharge.
For example, a lithium ion secondary battery has a configuration in which a cathode and a anode are connected through an electrolyte layer and are accommodated in a battery case. The electrolyte layer can be composed of a separator holding electrolyte solution. The separator needs to have both a function as a partition wall and a function to hold the electrolyte solution for ensuring the conduction of lithium ions between the cathode and anode. The above separator is usually composed of a microporous membrane made of an electrically insulating material.
Conventionally-developed separators have a shutdown function to stop charge/discharge reactions when the battery becomes hot during the charge/discharge reactions. The shutdown function shuts down the movement of lithium ions between the anode and cathode. To be specific, when the battery reaches a high temperature, the resin constituting the separator melts and clogs up the pores to shut down the movement of lithium ions. Accordingly, the separators which have the shutdown function are usually made of thermoplastic resin such as polyethylene (PE) or polypropylene (PP).
On the other hand, it is known that the separators made of the above thermoplastic resin have a problem with mechanical strength because of the flexibility of the materials. Under high-temperature conditions, in particular, a thermoplastic separator thermally shrinks, and the cathode and anode, which are opposed to each other with the separator interposed therebetween, could come into contact with each other, creating an internal short circuit. Accordingly, developments have been made in a technique to reduce thermal shrinkage due to heat treatment in the manufacturing process of batteries, reaction heat from the charge/discharge reactions, and the like.
For example, Patent Literature 1 discloses a porous membrane which includes a surface protecting layer containing inorganic particles of aluminum oxide or the like that is formed on at least one surface of a base made of PE or other such materials, that is, a separator. Furthermore, Examples of Patent Literature 1 describe that the separator has a low shrinkage ratio between before and after heat treatment.